1. Field of Invention
This application relates to liquid crystal electro-optical switching devices and, more particularly, relates to electro-optical switching devices employing ferroelectric liquid crystals.
2. Description of the Prior Art
The manipulation of light (electromagnetic radiation in the infrared, visible, and ultraviolet spectral regions) is becoming an important information handling technology. The construction of integrated optical circuits and the transmission of information along optical fibers are particularly attractive means for exploiting the information carrying capacity of light. A variety of devices, particularly for the generation and detection of intensity modulated light, have been developed for fiber and integrated optic systems. However, the development of complete optical systems has been hindered by the lack of suitable electro-optic (EO) switches for manipulating incident light by the application of electrical signals. Prior art EO switches are made from any material by which the intensity, polarization, or direction of the light can be electrically controlled, but the ease with which the molecular orientation and hence refractive index of liquid crystals (LCs) can be manipulated by applied electric fields has led to the development of liquid crystal electro-optic switching devices which exhibit improved light manipulating characteristics.
These electro-optic switching devices of the prior art have exploited a variety of electrically-induced molecular reorientation effects in nematic and chiral smectic liquid crystal phases, as will be described below.
The electro-optic effects in LCs result from electrical and optical anisotropies of the LC phases which in turn result from molecular ordering. In nematic liquid crystals (hereinafter referred to as nematics), the molecules tend to orient so that their average symmetry axes are parallel to a locally common direction. This direction defines the unit director field n,. (Note that n is commonly written herein and in the art as a vector, but since there is no physical significance attached to its sign, that is n, and -n, describe the same physical states, n, can be represented as a line segment.) Besides this orientational order, nematics are much like ordinary liquids. A consequence of this orientational order is anisotropy of the dielectric properties of nematics. Namely, although a linear relationship D=.epsilon.E still exists between the electric and displacement fields within a nematic, the dielectric constant .epsilon. is a second rank tensor. This causes the free energy of a nematic in an externally applied electric field to depend on its director orientation. If the director is not otherwise constrained, it will rotate to the orientation that minimizes the nematic's electrical free energy. Since the dielectric anisotropy extends to optical frequencies, this electrically-induced reorientation produces an electro-optic effect that may be exploited in practical light modulation devices.
The usual method for applying electric fields in these devices is to place the LC between closely-spaced parallel electrode plates. In this geometry, voltage applied across the plates produces an electric field perpendicular to the plates. Since the electrostatic energy ##EQU1## does not depend on the sign of E, this geometry allows only one field-preferred optical state. This state will have EQU n .parallel.E if .DELTA..epsilon.&gt;0, or n .perp.E if .DELTA..epsilon.&lt;0,
where EQU .DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp.
is the difference between the principal values of .epsilon. along axes parallel and perpendicular to n, respectively.
Since to be useful a device must have more than one optical state, some means must be found to prefer a director orientation other than the one preferred by the field. This is usually accomplished by treating the surface of the electrode plates such that they prefer a different orientation. Then, the applied field produces elastic strain in the orientation of n. When the electric field is removed, the stress resulting from that strain causes the orientation to relax back to that preferred by the surfaces. This means of operation has several consequences for the dynamic characteristics of such devices. For instance, while the turn-on time of such a device can be made arbitrarily short by increasing the applied electric field strength, the turn-off time is determined solely by geometrical size of the device, and may be undesirably long.
Over the past several years, another class of liquid crystals, ferroelectric liquid crystals (FLCs), has been developed. FLCs have the orientational order characteristic of nematics, as described above, and in addition have their molecules arranged in layers so that their mass density is quasiperiodic in one direction. Layered LC phases are called smectic phases. In smectic LCs (as opposed to ordinary solids) the distribution of the molecules within the layers is somewhat liquid in nature. The prerequisites for ferroelectricity occuring in smectics are that: (1) the constituent molecules must be chiral, in other words not superimposable on their mirror images; and (2) n must be tilted from the direction normal to the layer surfaces. When these conditions are met, the LC will have a spontaneous ferroelectric polarization P even in the absence of an applied electric field, as pointed out by Meyer et al. in Le Journal de Physique, Volume 36, pages L69-71, March, 1975.
The geometrical relationships between the layers of ferroelectric liquid crystals in the chiral tilted smectic phase, n, and P are shown in FIG. 1. In FIG. 1, the smectic layer planes are parallel to the X-Y plane and perpendicular to the z-axis. The director n in its preferred orientation tilts away from the z-axis by the angle .psi..sub.o. In addition, the projection of n onto the layer plane X-Y defines the "c-director" c, which makes an angle .phi. to the y-axis. Finally, the permanent polarization P is in the plane of the layers and perpendicular to n (i.e., P=P.sub.o z.times.n), that is, it is perpendicular to c, making the angle .phi. to the x-axis.
The displacement field D in FLCs has a dielectric part linearly proportional to E, similar to that described above in nematics, but in addition has a permanently nonzero part resulting from the spontaneous polarization P. Thus, the electrostatic free energy of FLCs has a part which is quadratic in E like nematics, and a part linear in E, proportional to -P.multidot.E. The FLC seeks to orient its director so as to minimize the total free energy within its thermodynamic constraints, which constraints essentially fix the tilt angle .psi..sub.o. Flow involving changes in the direction of the layering is extremely dissipative, however, leaving changes in the azimuthal angle .phi. as the principal means available to the FLC to minimize its free energy. The inclusion of a term proportional to -P.multidot.E makes this energy depend on the sign of an applied electric field. Thus, practical devices can be made in the above-mentioned parallel plate geometry where two optically distinct states may be selected by voltages of opposite sign applied to the plates. This allows both the turn-off and turn-on times of such a device to be made shorter by increasing the applied field strength.
The dielectric anisotropy of both nematics and FLCs results in them being optically anisotropic, with the nematics being uniaxial and the FLCs being biaxial, but with refractive indices along the two axes perpendicular to n typically nearly equal, so that for most purposes FLCs are considered to be optically uniaxial, with the optic axis along the director n as in nematics. Thus, in either case, electrically induced orientation changes produce changes in refractive index, and these electro-optic effects may be exploited for practical devices, despite the substantial differences in the geometry of the optic axis and applied field directions.
One class of such devices makes use of the change in polarized light passing through an "optically thick" slab of LC material. By "optically thick" is meant that .DELTA.nd is comparable or larger than .lambda., where .DELTA.n is the LC refractive index anisotropy (birefringence), d is the slab thickness, and .lambda. is the light's vacuum wavelength. The twisted nematic, supertwisted birefringence effect, nematic .pi.-cell, and variable birefringence nematic devices all fall in this class, as does the family of surface-stabilized FLC (SSFLC) devices proposed by N. Clark and S. Lagerwall in U.S. application Ser. No. 511,733, now U.S. Pat. No. 4,563,059, and U.S. application Ser. No. 797,021. These devices operate best when the light they are modulating is incident in a direction near to the normal of the slab. Typically, such a device is placed between a crossed polarizer and analyzer, and it operates so that in one state the transmitted output light is greatly attenuated in intensity by being mostly if not completely absorbed in the analyzer, whereas in the other state the output light is transmitted with as much of the incident intensity as is practical. In neither state does the direction of the incident light propagation change upon crossing the LC layer. Of course, variations are possible where reflective materials are incorporated into such devices such that the incident light is returned toward its source after two passes through the LC slab, but the basic action is still as described above.
Another class of such devices relies on the reflection and refraction properties of light at an interface between two dissimilar dielectrics, one of which is an LC. Examples in the prior art of this class of devices using nematic LCs are disclosed in U.S. Pat. Nos. 4,201,442 and 4,278,327 by D. H. McMahon and R. A. Soref and U.S. Pat. No. 4,385,799 to Soref. Also, as taught by Kashnow and Stein in Applied Optics, Vol. 12, No. 10, October, 1973, electrooptic effects have been achieved by placing a thin nematic liquid crystal layer between two glass prisms of appropriate refractive index. For a range of angles of incidence of light on the prism-liquid crystal interface, the light is partially transmitted or totally reflected, depending upon the electric field controlled orientation of the optic axis in the nematic layer. Furthermore, as disclosed by Terui and Kobayashi in Proceedings of the SPIE, Vol. 517, p. 267 (1984), the light need not propagate through the FLC medium itself for this type of switching to work, for the same total internal reflection phenomena is obtained where the LC material merely forms the "cladding" for a waveguide of some higher index material. These inventions thus relate to electro-optic devices for switching input light more or less completely between two or more outputs. They separate the outputs by changing the direction of the input light by reflection at an interface between a nematic LC and another dielectric. Consequently, they teach that by selectively applying electric fields to the nematic LC, the direction of the output light may be selected.